Thermal pulses of various amplitudes will be applied to the hindlimbs of cats and hands and tails of monkeys being fed according to an automated program which controls the timing and intensity of stimuli. The stimulus-associated probabilities and latencies of 3 responses of cats will be measured: movement, vocalization, and interruption of eating. Monkeys will press a contact thermode to obtain food; the probability and latency of thermode release and tail movement will be determined for each thermal stimulus intensity. Noxious stimuli elicit these responses significantly more often than blank or innocuous stimulus trials. This behavioral testing system will be used to: 1) identify, by means of selective lesions, spinal and supraspinal pathways for thermal pain; and 2) test, by means of lesions and electrical stimulation, the hypothesis that certain brainstem raphe nuclei form a descending pain suppression system which is active under normal physiological conditions.